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Demystifying Silica Gel

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Demystifying Silica Gel DEMYSTIFYING SILICA GEL Steven Weintraub ABSTRACT - It is important to understand how silica gels vary in performance in order to select the most cost-effective gel for a particular application. MH, the hysteresis corrected buffering capacity of silica gel, is the critical variable for assessing silica gel efficiency. Calculating the correct quantity of silica gel allows for the cost-efficient selection of an appropriate amount of buffering material. If certain variables in the calculation are unknown, such as leakage rate or external RH conditions, general recommendations based on average display conditions have been provided, both for temporary exhibitions and for permanent displays. Finally, simple procedures for the use and maintenance of silica gel have been described. Passive humidity control within an exhibit case, when applied correctly, is a very simple and cost-efficient method of protecting museum collections from humidity induced damage. 1. INTRODUCTION In 1959, silica gel was first recommended for use in museum applications as a buffering agent to control relative humidity (RH) in “closed packages” (Toishi 1959). Since that time, in spite of silica gel’s use for museum exhibition case RH control throughout the world, there has been a great deal of mystery and confusion regarding the use of silica gel systems. The purpose of this article is to demystify and explain basic information about silica gel: • How does silica gel function? • What are the significant differences among silica gels used in museum applications? • How much silica gel is required to control relative humidity in an exhibition case? • How can silica gel be reconditioned? 2. WHAT IS SILICA GEL AND HOW DOES IT WORK? 2.1 EMC/RH ISOTHERM In order to understand how silica gel functions, it is critical to understand the concept of Equilibrium Moisture Content (EMC). Many materials contain moisture. The quantity of moisture in hygroscopic materials depends on the temperature and RH of the surrounding air. If the temperature or RH changes, the moisture content within the object will change so that it will come into equilibrium with the new condition of the surrounding air. Moisture content is the weight of water in an object expressed as a percentage of its dry weight. The EMC is the moisture content of an object in equilibrium with a specified RH. For example, if a piece of paper weighing 100 grams at 0% RH increases to 105 grams at 50% RH, it now has 5 grams of moisture compared to its dry weight, resulting in a 5% EMC at 50% RH: (105 g at 50% RH – 100 g at 0% RH)/100 g (dry weight) = 0.05 = 5% EMC 1 • To understand the moisture uptake characteristics of hygroscopic materials, a series of EMC values for the full range of RH conditions at a fixed temperature can be plotted. This is known as an EMC/RH isotherm (Fig. 1). Figure 1. Equilibrium Moisture Content / Relative Humidity Isotherm 35 30 25 Regular Density Gel ) 20 Artsorb Cotton/Linen/Paper EMC (% 15 Wood 10 Wool 5 0 0 20406080100 RH (%) For organic materials, it is theoretically important to take temperature into account since it affects EMC. But, in actuality, a moderate change in temperature has a relatively small influence on EMC compared to a moderate change in RH (temperature has no effect on the EMC of silica gel within the normal range of museum use). Because hygroscopic objects are far less affected by temperature than RH in terms of impact on the moisture content and physical stability, RH is the principle focus of concern. And, since RH rather than the absolute moisture in air determines the moisture content of an object, we are concerned with RH rather than absolute humidity. [1] 2.2 BUFFERED CASES AND RH CONTROL The interior of a relatively airtight exhibition case will provide some level of protection against fluctuating RH conditions outside the case. However, through gradual air leakage, the RH inside the case will slowly increase or decrease, depending on the condition of the outside RH. The rate of interior RH change depends on the amount of leakage. If the rate of air leakage is one air exchange per day, the RH within the case will equal the RH of the surrounding air within one day. [2] In reality, if there are a lot of hygroscopic materials within the case, the interior RH of a case with a leakage rate of one air exchange per day will barely change. The reason for this is the buffering effect of the hygroscopic materials within the case. • As the exhibition case gains or loses humidity because of leakage, the hygroscopic materials within the case must gain or lose some moisture content in order to remain in equilibrium with the RH of the surrounding air. The water gained or lost by these materials offsets most of the expected change in RH within the case. In effect, these materials act as buffers to slow down the rate of change in RH within the exhibition case. 2 For example, a one meter case with an internal RH of 50%, an external RH of 25% and an air exchange rate of 1x per day will lose 5 grams of moisture in a day as the internal RH decreases from 50% to 25% RH (there are about 10 g/m³ at 50% RH and 5 g/m³ at 25% RH at 22.7ºC). However, if the case contains a great deal of hygroscopic materials, these materials will give off some of their moisture as the RH within the case falls in order to remain in equilibrium with the surrounding RH. As a consequence, the moisture given off by these materials offsets almost all of the 5 grams of moisture in air lost through leakage, so the RH in the case only falls by a fraction of a percent rather than by 25% RH over a single day. 2.3 BUFFERING CAPACITY AND RH CONTROL As a result of the buffering effect of hygroscopic materials, the RH within an exhibition case will show only a very small daily fluctuation or a very slow change in RH over time compared to conditions outside the case (Fig. 2). However, the degree of internal fluctuation or change in RH within the case depends on the buffering capacity of all the hygroscopic materials within the case. If the case has a relatively small ratio of hygroscopic materials compared to the total case volume, the buffering capacity of the case will be limited compared to a case with a large amount of hygroscopic materials. • In practical terms, this means that a case with a large amount of buffering capacity may take many months for the RH to decrease from 50% RH to 25% RH, whereas if there is very little buffering capacity, the decrease may occur over a period of days or weeks. In fact, for an exhibition case where the only hygroscopic material is the object itself, the object becomes the buffer. By introducing other buffering materials into the case, it is possible to significantly reduce the rate of change of moisture from the object itself, thereby reducing risk of RH induced damage. [3] Figure 2. Annual Climatic Cycle Annual Climatic Cycle 80 60 40 RH (%) 20 0 12345678910111213 Time (mont hs) in the room in the exhibit case 3 Since all hygroscopic materials provide some level of buffering capacity, why do museums use silica gel for this purpose rather than inexpensive, easily available organic materials like cotton? The primary reason is because of the much higher buffering efficiency of silica gel compared to organic materials. [4] Buffering capacity is based on the amount of moisture that a material will gain or lose within a specified range of RH. The buffering capacity of materials can be compared in a general way by looking at their EMC/RH isotherms (Fig. 1). From this graph, it is clear that the two illustrated silica gels have the capacity to adsorb much more moisture than natural materials such as wood, cotton or wool at the low to mid-RH range. As a consequence of silica gel’s high moisture capacity, far less silica gel is needed by weight to achieve a certain amount of buffering capacity compared to organic materials. In addition, because of the high density of silica gel (approximately 0.7 kilograms per liter or 44 pounds per cubic foot for a regular density grade), it takes up far less space in an exhibition case than an organic material with equivalent buffering capacity. 2.4 SILICA GEL – A BRIEF DESCRIPTION AND HISTORY Silica gel is a chemically inert, non-toxic material composed of amorphous silicon dioxide. It has an internal network of interconnecting microscopic pores, yielding a typical surface area of 700- 800 square meters per gram; or, stated another way, the internal surface area of a teaspoon full of silica gel is equivalent to a football field. Water molecules are adsorbed or desorbed by these micro-capillaries until vapor pressure equilibrium is achieved with the relative humidity of the surrounding air. Silica gel was patented in 1919 for use in the adsorption of vapors and gases in gas mask canisters during World War I. During World War II, it was commonly used as a dehydrating agent to protect military and pharmaceutical supplies, among a number of other applications. Its use as a buffering agent to control RH within the mid-range rather than as a desiccant is a unique to museum applications. 3. DIFFERENT TYPES OF SILICA GEL – DIFFERENT TYPES OF PERFORMANCE 3.1 DEFINING BUFFERING CAPACITY – THE VARIABLE M The moisture adsorbing properties of silica gels are affected by factors such as capillary pore size or the inclusion of hygroscopic salts, resulting in a wide range of performance.
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